Semiconductor laser and the method for manufacturing the same

ABSTRACT

The present invention is to provide a semiconductor laser with a feedback grating comprised of InP and AlGaInAs without InAsP put therebetween, and to provide a method for manufacturing the DFB-LD having such grating. The LD includes an n-type InP substrate, an AlInAsP intermediate layer, an AlGaInAs lower SCH layer, an active layer, and a p-type layer for upper cladding in this order from the InP substrate. The InP substrate, the AlInAsP intermediate layer, and the AlGaInAs lower SCH layer constitute the feedback grating. The AlInAsP intermediate layer lowers a series resistance along these semiconductor stacks.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of Ser. No. 11/324,800, filed Jan. 4,2006 now U.S. Pat. No. 7,460,575 and which is being incorporated in itsentirety herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a structure of a semiconductor laserdiode and a method for producing the laser diode.

2. Related Prior Art

Japanese Patent published as A-H11-112098 has disclosed a distributedfeedback (DFB) laser formed on the InP substrate. On the InP substratewith a feedback grating is grown in sequence the guiding layer, thespacer layer, and the MQW layer. The temperature profile to grow theselayers shows a steep rise within 2 minutes and without any overshootingto the condition under which each layer is grown and makes thetemperature drift stable within ±5° C. Moreover, the InGaAsP guidinglayer starts its growth before the mass transportation of indium atomsoccurs. Thus, the method disclosed in this patent prevents thedecomposed InAsP layer from piling in the valley portion of the gratingwhen the double hetero structure for the DFB-LD is formed on the InPsubstrate with the grating by the Organic Metal Vapor Phase Epitaxy(OMVPE) method.

The background of the present invention is that, in the DFB-LD, to keepthe shape of the feedback grating is quite important for the single modeemission. When the InP substrate with the undulation for the feedbackgrating on the surface thereof is raised in the temperature as supplyingthe gas sources for the group V element, such as arsine AsH₃ andphosphine PH₃, an InAsP decomposed layer may be piled in the valleyportion of the undulation, which degrades the shape of the feedbackgrating. The refractive index necessary to diffract light may be unableto obtain, consequently, the single mode oscillation can not be secured.Therefore, various methods has been proposed to maintain the dimensionalquality of the feedback grating, in particular, the flow rate of the gassource for the group V element may be adjusted during the raising of thegrowth temperature.

When a semiconductor layer is grown immediately on the InP substratewith the grating on the surface thereof by the OMVPE method, the layerordinarily called as the guide layer, first, the InP substrate is raisedin the temperature thereof under an atmosphere including the group Velement. The practical growth of the guide layer does not start untilthe temperature of the InP substrate becomes satisfactorily stable. Thisis because an overshoot of the temperature, i.e., the temperature of thesubstrate exceeds the growth temperature, occasionally appears in arange from several decades to around a hundred centigrade. Therefore, itis necessary to set a waiting time of about 10 minutes for thetemperature being stable enough, and by adjusting the flow rate of thegas source of the group V element during this waiting, the dimensionalquality of the grating may be secured.

The semiconductor material system for the DFB-LD of the presentinvention includes a separated confinement hetero-structure (SCH) layermade of AlGaInAs, which is quite different from the conventional systemmade of InGaAsP/InP. When this SCH layer is grown on the substrate withthe grating, an intermediate layer of the InAsP is formed to secure thedimensional shape of the grating during the raising of the temperatureas supplying the phosphine PH₃ containing a minute amount of the arsineAsH₃. The grating is comprised of the InP substrate, the InAsPintermediate layer, and the AlGaInAs SCH layer. However, the seriesresistance of this DFB-LD along the stacking direction of semiconductorlayers does not show an anticipated resistance. This may be due to theband structure between the InAsP and the AlGaInAs. Therefore, an objectfor the DFB-LD including AlGaInAs material is an innovative bandstructure to overcome the subject between the InAsP and the AlGaInAs.

The present invention, performed to solve the above subject, is toprovide a semiconductor laser diode with a feedback grating comprised ofthe AlGaInAs and the InP without the InAsP, and to provide a method formanufacturing such laser diode with a new structure.

SUMMARY OF THE INVENTION

One aspect of the present invention relates to a configuration of asemiconductor laser diode (LD) with a feedback grating. The LD comprisesa substrate with an undulation for the feedback grating, a lowerseparated confinement (SCH) layer, an active layer, and an upper SCHlayer stacked in this order on the substrate. The LD of the inventionfurther includes an intermediate layer between the substrate and thelower SCH layer. The substrate, the intermediate layer, and the lowerSCH layer constitute the feedback grating, and may be an n-type InP, anAlInAsP, and an AlGaInAs, respectively.

Since the AlInAsP intermediate layer may prevent the dimensional qualityof the undulation during the growth of the lower SCH layer fromdegrading, the optical coupling efficiency between the grating and theactive layer may be secured, consequently, the single mode oscillationof the LD may be maintained. Moreover, the band structure of thesesemiconductor materials, in particular, the relation between the bottomlevels of each conduction band ensures the resistance along the stackingof layers to be small. That is, the bottom level of the conduction bandof the AlInAsP intermediate layer is higher than that of the InPsubstrate and, at the same time, is lower than that of the AlGaInAslower SCH layer, which makes no valleys nor peaks in the conductionband. Accordingly, the resistance thereof may be suppressed.

The AlInAsP intermediate layer may have a thickness thinner than 10 nmsuch that the existence of the AlInAsP intermediate layer between theInP substrate and the AlGaInAs lower SCH layer does not affect theoptical structure therebetween, in particular, the refractive indicesthereof. Moreover, The band gap wavelength of the AlGaInAs lower SCHlayer is preferably longer than 1.2 μm for the structure of bottomlevels of the conduction band of each layer.

Another aspect of the invention relates to a method for manufacturingthe LD with the feedback grating. The method comprises steps of; (a)forming an undulation on a surface of the InP substrate, (b) epitaxiallygrowing an AlInAsP intermediate layer onto the undulated surface of theInP substrate, and (c) epitaxially growing a lower SCH layer on theAlInAsP intermediate layer. The epitaxial growth of the AlInAsPintermediate layer may be carried out by supplying with a mixed gas ofarsine, phosphine, and organoaluminum.

The method may further include, after forming the undulation on thesurface of the substrate and before growing the intermediate layer, astep for raising temperature of the substrate to a first preset value assupplying with hydrogen, and subsequently, to a second preset value,which is higher than the first value, as supplying with hydrogen, andphosphine, until the temperature of the substrate sufficiently stable.Accordingly, the dissociation of phosphorus atoms from the surface ofthe InP substrate may be prevented. The growth of the AlInAsPintermediate layer may be carried out by supplying with arsine,phosphine, and organoaluminum at the second preset temperature.

The substrate grown with the AlInAsP intermediate layer on theundulation of the surface thereof may be raised in the temperaturethereof to a third preset value, which is higher than the second presetvalue, as supplying with hydrogen, arsine, and phosphine. The growth ofthe AlGaInAs layer may be carried out at the third temperature. Sincethe AlInAsP intermediate layer is provided on the surface of the InP,the mass-transportation of indium atoms may be prevented, therebymaintaining the dimensional quality of the undulation for the feedbackgrating and the optical coupling efficiency between the grating and theactive layer formed on the grating. Accordingly, the LD may secure thesingle mode oscillation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial cross section showing a structure of the laser diodeaccording to the first embodiment of the invention;

FIG. 2A shows an energy band diagram of the InP/InGaAsP system, whichconstitutes the feedback grating of the laser diode,

FIG. 2B is an energy band diagram of the InP/AlGaInAs system, and

FIG. 2C is the band diagram of the laser diode shown in FIG. 1;

FIG. 3A shows a process to raise the temperature of the growth chamber,which prevent phosphorus atoms from dissociating from the InP surface,FIG. 3B shows a process to raise the temperature, which prevents themass transportation of indium atoms, and FIG. 3C shows a process toraise the temperature after forming the intermediate layer;

FIG. 4A shows a process for epitaxially growing a stack of semiconductorfilms, FIG. 4B shows a process for forming the mesa stripe, and FIG. 4Cshows a process to bury the mesa stripe;

FIG. 5A shows a process for forming the contact layer, and FIG. 5B showsa process for forming electrodes; and

FIG. 6 is a time chart showing a sequence of supplying gas sources.

DESCRIPTION OF PREFERRED EMBODIMENTS

The spirits of the present invention will be easily understood by takingthe following description into account as referring to accompanyingdrawings. Next, an embodiment according to a structure of asemiconductor optical device of the present invention and a method formanufacturing the optical device will be described as referring todrawings. In the explanation below, a semiconductor laser diode(hereinafter denoted as LD) is raised as an example of the semiconductoroptical device. If possible, the same numerals or the same symbols willrefer the same elements without overlapping explanations.

First Embodiment

FIG. 1 is a partial sectional view of a semiconductor laser diode (LD)according to the first embodiment of the present invention. The LD 11includes an n-type InP region 13, a p-type III-V compound semiconductorlayer 15, an upper separated confinement (SCH) layer 25, an AlGaInAslayer 17, an AlInAsP layer 19, which operates an a lower SCH layer, andan active layer 21. The AlInAsP layer 19 is formed between the n-typeInP region 13 and the AlGaInAs layer 17. The active layer 21 is formedbetween the upper SCH layer 25 and the AlGaInAs layer 17. The n-type InPregion 13, the AlInAsP layer 19, and the AlGaInAs layer 17 constitute adistributed feedback grating 23. This feedback grating 23 opticallycouples with the active layer 21.

As explained later, the LD 11 has a band structure that a differencebetween the bottom level of the conduction band of the AlGaInAs layer17, which is denoted as E_(C) ^((AlGaInAs)) and that of the AlInAsPlayer 19, denoted as E_(C) ^((AlInAsP)), is smaller than a differencebetween the bottom level of the conduction band of the InAsP, which isdenoted as E_(C) ^((InAsP)) and ordinarily appeared in the convention LDwith the feedback grating, and that of the InP region, E_(C) ^((InP)).

To form the feedback grating 23, it is necessary to make an undulationon the surface of the InP region 13. By disposing the AlInAsP layer 19onto the undulation of the InP region 13, the mass-transportation ofindium atoms may be prevented during the raising of the temperature forthe next growth of the AlGaInAs layer 17. Consequently, the AlGaInAslayer 17 is formed onto the AlInAsP layer 19 with high quality. Then-type InP region 13 includes an n-type InP substrate 13 a and an n-typeInP layer 13 b provided on the n-type InP substrate 13 a.

FIG. 2A shows a band structure of the semiconductors constituting thefeedback grating in the InP/InGaAsP system, which is a typicalcombination in the conventional LD, FIG. 2B is that of the InP/AlGaInAssystem, while, FIG. 2C is the band structure of semiconductorsconstituting the feedback grating according to the present invention.

When the lower SCH layer is grown, by using the Organic Metal VaporPhase Epitaxy (OMVPE) method, onto the InP region with a mask patternfor the feedback grating, the growth temperature is raised as supplyingwith the phosphine (PH₃) to prevent the dissociation of phosphorus atomsfrom the surface of the InP region. However, when the temperature israised as supplying only with the phosophine, the mass-transportation ofindium atoms may occur and the feedback grading on the surface of theInP region can not be maintained. Accordingly, the conventional methodfor forming the feedback grating uses a mixture of the phosphine and thearsine (AsH₃) during the raising of the temperature. That is, in theconventional structure, the InAsP reformed layer is naturally formed onthe InP region as supplying with the mixture of the phosphine and thearsine. Conventionally, the InGaAsP layer is grown thereon to form thefeedback grating 23. The method of growing the InGaAsP layer assupplying with the mixture of the phosphine and the arsine prevents theresistance of the InGaAsP layer along the stack of the layers fromincreasing.

However, in the case that the feedback grating uses the AlGaInAs lowerSCH layer substituting for the InGaAsP of the conventional structure,and the AlGaInAs layer is grown after the raising of the temperature assupplying with the mixture of the arsine and the phosphine, and theInAsP layer is naturally grown, the relation of the bottom level of theconduction band between the InAsP layer and the AlGaInAs layer becomesunfavorable for the carrier conduction.

As shown in FIG. 2B, since the bottom level of the conduction band ofthe InAsP layer, E_(C) ^((InAsP)), becomes lower than that of the InPregion, E_(C) ^((InP)), a difference δE1 of the bottom levels of theconduction band between the InAsP layer, E_(C) ^((InAsP)), and theAlGaInAs layer, E_(C) ^((AlGaInAs)), increases. On the other hand in thepresent invention, the surface of the InP region is covered by theAlInAsP layer during the raising of the growth temperature by supplying,in addition to the arsine and the phosphine, an aluminum containingsource such as trimethylaluminum (TMAl) or triethylaluminum (TEAl).After covering the surface of the InP region by the AlInAsP intermediatelayer, the AlGaInAs is grown. Since the AlInAsP is applied substitutingfor the InAsP, the band structure of semiconductor layers, especiallythat of the bottom level of the conduction bands, becomes that shown inFIG. 2C. That is, the bottom level of the conduction band of theAlInAsP, E_(C) ^((AlInAsP)), may be close to that of the InP, E_(C)^((InP)), and the former level, E_(C) ^((AlInAsP)), may be higher thanthe latter level, E_(C) ^((InP)), which decreases the difference of thebottom levels of the conduction band between the AlGaInAs, E_(C)^((AlGaInAs)), and the AlInAsP, E_(C) ^((AlInAsP)).

As shown in FIG. 2C, the band structure according to the presentinvention, the bottom level of the conduction band of the AlInAsP layer,E_(C) ^((AlInAsP)), lies between that of the AlGaInAs layer, E_(C)^((AlGaInAs)), and that of the InP region, E_(C) ^((InP)). Accordingly,the resistance inherently attributed to the band discontinuity δE₁between the InP region 13 and the AlInAsP layer 19, as well as that,δE₃, between the AlInAsP layer 19 and the AlGaInAs 17 layer may besuppressed.

The thickness of the AlInAsP layer 19 may be thinner than 10 nm, whichdoes not affect the optical structure between the AlGaInAs layer 17 andthe InP region 13, in particular the refractive indices thereof.

Moreover, the band gap wavelength of the AlGaInAs layer 17 may be longerthan 1.2 μm. Here, the band gap wavelength corresponds to the energy ofthe fundamental absorption edge of the semiconductor materials. When theband gap wavelength is shorter than 1.2 μm, the band structure betweenthe InP region 13 and the AlGaInAs layer 17 may be preferable, inparticular, the relation between the bottom levels of the conductionband in each layer.

Referring to FIG. 1 again, the LD 11 includes a mesa stripe 27comprising the p-type III-V layer 15, the AlGaInAs lower SCH layer 17,the AlInAsP layer 19, the active layer 21, and the upper SCH layer 25.The mesa stripe 27 is buried by a semiconductor region 29 that may be anInP doped with iron (Fe). On the mesa stripe 27 as well thesemiconductor region 29 is formed with an upper cladding layer 31 and acontact layer 33 each made of p-type III-V compound semiconductormaterial. The former layer 31 may be made of a p-type InP, while thecontact layer 33 may be made of a p-type InGaAs. An anode electrode 35is formed on the contact layer 33, while in a back surface of the InPregion 13 is provided with a cathode electrode 37.

The LD 11 of the present invention forms the AlInAsP layer 19,substituting the InAsP intermediate layer in the conventionalconfiguration, to smoothly connect the band structure of the InP regionto the AlGaInAs layer, which suppresses the increase of the inherentresistance of the semiconductor stacking. The practical resistance ofthe stack shown in FIG. 2C was 6Ω, while that of FIG. 2B was 2Ω at 25°C. Moreover, the AlInAsP layer is grown on the undulated surface of theInP region, the mass-transportation of indium atoms, which may occur onthe surface of the InP, may be prohibited and, accordingly, the feedbackgrating may be secured in its undulated shape.

Second Embodiment

From FIGS. 3A to 3C, from FIGS. 4A to 4C, and FIGS. 5A and 5B showprocesses to form the LD of the present embodiment shown in FIG. 1.

At a step shown in FIG. 3A, the InP substrate 43 that forms anundulation for the feedback grating is set within a growth chamber 41.Raising the temperature of the substrate 43 as supplying only with thehydrogen gas (H₂) into the chamber 41, and when the temperature is closeto a condition where the dissociation of the phosphorus atoms from thesurface of the InP substrate 43, the phosphine (PH₃) in addition to thehydrogen is supplied within the chamber 41 to prevent the dissociationthereof. Further raising the temperature and the temperature approachesa condition that the mass-transportation may occur, the arsine (AsH₃)and an organoaluminum source, for instance TMAl, are started in additionto the hydrogen and the phosphine to growth the surface of the InPsubstrate 43 and to obtain the intermediate layer of the AlInAsP, asshown in FIG. 3B. After growing the layer 45, the TMAl is stopped andthe temperature is raised to a condition to grow the AlGaInAs—SCH layeras supplying with the mixture of the hydrogen, the phosphine, and thearsine.

The AlGaInAs SCH layer 47 is grown to from the feedback grating afterthe temperature of the chamber reaches the preset condition and becomessufficiently stable. After the growth of the AlGaInAs—SCH layer 47, thetemperature is further raised to a condition under which the activelayer is grown. After the condition is obtained, another AlGaInAs forthe active layer 49 is grown. Subsequently, the upper SCH layer 51 madeof AlGaInAs, and the p-type InP upper cladding layer 53 are grown insuccessive under respective optimal growth condition.

Next, as shown in FIG. 4B, a stack of semiconductor layers from 43 to 53is etched by using an etching mask 55 to form the mesa stripe 57. Themesa stripe 57 includes a portion of the InP substrate 43 a, the AlInAsPlayer 45 a, the AlGaInAs lower SCH layer 47 a, the AlGaInAs active layer49 a, the AlGaInAs upper SCH layer 51 a, and the p-type upper claddinglayer 53 a.

Both sides of the mesa stripe 57 is buried by the semiconductor material59 without removing the etching mask 55 after the semiconductor stack,the substrate 43 a with the mesa stripe 57, is put back within thegrowth chamber 41. After the growth of the side regions 59, the etchingmask 55 is removed.

Loading the substrate 43 a with the mesa stripe 57 and the side regions59 into the chamber 41 again, a part of the upper cladding layer 61,which may be made of the p-type InP, and the contact layer 63 are grownas shown in FIG. 5A. Two electrodes of the anode 67 and the cathode 69are formed on the contact layer 63 and the bottom surface of the InPsubstrate 43 a, respectively. The anode 67 is formed via a passivationfilm 65 made of insulating material such as silicon nitride (SiN) withan opening from which the surface of the contact layer 63 is exposed.

Referring to FIG. 6, a sequence of the epitaxial growth of the presentinvention, in particular the sequence of the supply of gaseous sourceswill be described. First, the InP substrate with the undulated structurefor the feedback grating is loaded into the growth chamber of the OMVPEapparatus. The pressure of the growth is 60 Torr (8000 Pa). At the timet0, the temperature of the chamber is raised to 400° C. as supplyingwith the hydrogen H₂. The flow rate of the hydrogen may be, for example,10 slm (standard litter per minutes). At 400° C., the phosphine (PH₃) issupplied in addition to the hydrogen (H₂) within the chamber at thetiming t1 to control the dissociation of the phosphorus (P) from the InPsubstrate. The flow rate of the phosphine (PH₃) may be, for example, 50sccm (standard cc per minutes). At the temperature being 450° C., thearsine (AsH₃) and the TMAl are further supplied within the chamber atthe time t2 to grow the AlInAsP intermediate layer with a preferablethickness. The flow rate of the arsine (AsH₃) is, for example, 0.5 sccm,while that of the TMAl is, for example, 20 sccm. In the presentembodiment, the mixed gas of the hydrogen (H₂), the phosphine (PH₃), thearsine (AsH₃), and the TMAl is supplied for about 3 seconds from t2 tot3 in FIG. 6, to obtain the intermediate layer of the AlGaInAs with athickness of about 1 nm. The TMAl is ceased its supply at the time t3.Subsequently, the temperature is raised to 550° C. from t3 to t4 assupplying a mixture of the phosphine PH₃ and the arsine AsH₃. At 550°C., the AlGaInAs layer is grown from t4 to t5. Subsequently, thetemperature is raised again from t5 to t6 to 700° C. as supplying onlythe arsine (AsH₃) to grow the AlGaInAs active layer at 700° C. After theAlGaInAs active layer is grown from t6 to t7 at 700° C., the temperatureis fallen to 670° C. from t7 to t8 as supplying only with the arsine(AsH₃). At 670° C., the InP cladding layer is grown from t8 to t9.

Although the present invention has been fully described in conjunctionwith the preferred embodiment thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

1. A method for manufacturing a semiconductor laser diode with afeedback grating, comprising steps of: forming an undulation on asurface of a semiconductor substrate; epitaxially growing anintermediate layer onto the undulated surface of the substrate so as toleave the undulation; and epitaxially growing a semiconductor layer tobury the undulation formed on the surface of the substrate to make asurface of the semiconductor layer substantially flat.
 2. The methodaccording to claim 1, wherein the substrate, the intermediate layer, andthe semiconductor layer are InP, AlInAsP, and AlGaInAs, respectively. 3.The method according to claim 2, wherein the step of forming the AlInAsPintermediate layer is carried out by supplying arsine, phosphine, andorganoaluminum.
 4. The method according to claim 1, wherein, afterforming the undulation and before growing the intermediate layer, thetemperature of the substrate is raised to a first preset temperature assupplying hydrogen, and subsequently raised to a second presettemperature higher than the first preset temperature as supplyinghydrogen and phosphine.
 5. The method according to claim 4, wherein thegrowth of the intermediate layer is carried out at the second presenttemperature as supplying hydrogen, phosphine, arsine and organoaluminum.6. The method according to claim 4, wherein, after growing theintermediate layer and before growing the semiconductor layer, thetemperature of the substrate is raised to a third preset temperaturehigher than the second preset temperature as supplying hydrogen,phoshine, and arsine.